1. Field of the Invention
This invention relates to cathode ray tubes. More particularly, the invention relates to an array of independently modulated electron beams emanating from field emission cathodes in a cathode ray tube.
2. Description of Related Art
Higher brightness and simpler construction have long been the goal of manufacturers of cathode ray tubes (“CRTs”). In commercial CRTs, brightness is limited by the electron gun current that can be extracted from a thermionic cathode and passed through a small aperture. The small aperture is required to restrict the electron source size and produce a small spot of light on a phosphor screen.
Another limitation on brightness of cathode ray tubes arises from the use of a shadow mask. The shadow mask is effectively a limiting aperture for the electron beam near the plane of the CRT screen. It normally captures from about 30 to 80 percent of the incident electron beam and reduces image brightness by a corresponding amount. Elimination of the shadow mask could regain this lost electron beam current and thereby increase image brightness. Elimination of the shadow mask would also reduce complexity of CRT manufacturing and thereby reduce cost.
Replacing the thermionic oxide cathode with a field emission (FE) cathode can increase electron current and brightness because the FE cathode can deliver a higher current density per unit area. Use of carbon-based FE cathodes in electron guns of CRTs has been disclosed in commonly assigned and co-pending application U.S. Ser. No. 09/169,908, filed Oct. 12, 1998 and in U.S. Pat. No. 6,181,055B1, which are hereby incorporated by reference herein. Still, even with a FE cathode, a brightness increase by a factor of two or three above an oxide cathode CRT is about the limit of achievability. This limitation is primarily caused by excessive space charge and phosphor power-dissipation limitations.
There is a significant quantity of prior art relating to multiple electron beams in CRTs. Most of the prior art is meant for use with thermionic cathodes. For example, U.S. Pat. Nos. 3,943,281, 4,954,901 and 5,557,344 disclose multiple-beam, multiple-raster CRTs that have a plurality of controllable cathodes that are oriented vertically. Each beam is directed to scan a separate scan line and is separately modulated, so that the horizontal scan rate can be reduced by a factor equal to the number of beams. This results in increased frame brightness. These patents all disclose a system requiring a shadow mask. U.S. Pat. No. 5,585,691 describes a method for changing video signal timing in order to provide dynamic color separation and remove the need for a shadow mask in a CRT. The method uses complicated electronics and is limited to specific applications.
There is a need for improved apparatus and method for increasing the brightness, obtaining better spot resolution, allowing instantaneous electronic beam turn-on and eliminating the requirement for a shadow mask in cathode ray tubes. Reductions in complexity and cost of manufacturing are also needed.